1. Field of the Invention
The present invention relates to a nonvolatile memory device, and particularly to a nonvolatile memory device operable with a low voltage.
2. Description of the Background Art
In accordance with progress of miniature processing technologies in recent years, cell sizes as well as sizes of transistors (film thicknesses of oxide films and others) have been reduced for reducing sizes of chips. However, a memory device uses different operation voltages for executing various operations. Therefore, it is difficult to employ such a structure that all circuit regions are formed of transistors, which have oxide films of small film thickness and thus small sizes, and may be referred to as “thin film transistors” hereinafter. In particular, a flash memory and others use various operation voltages in various operation sequences, and therefore generally have structures, in which memories subjected to high voltages or the like use transistors having large film thickness and relatively large sizes. These transistors may be referred to as “thick film transistors” hereinafter. Sense amplifiers and others, which must perform fast operations, use transistors capable of fast operation with a low operation voltage, and thus use transistors having small film thickness and relatively small sizes. This can reduce whole chip sizes. Various manners have been proposed for reducing transistor sizes and thereby reducing chip sizes. Japanese Patent Laying-Open No. 8-329690 has disclosed a nonvolatile memory device, in which a well structure of a memory cell is improved to lower an operation voltage of the memory cell, and thereby film thicknesses of transistors in peripheral circuits can be reduced.
However, a chip including two kinds of transistors as described above, i.e., a chip formed of thin film transistors and thick film transistors must be controlled to prevent application of a high voltage to the thin film transistors.
Particularly, in a flash memory, voltages at various levels are supplied to memory cells and peripheral circuits depending on various operation sequences, and therefore it is necessary to prevent breakage of the thin film transistors having small film thicknesses when an operation voltage is high.